Process for making aluminumsilicon alloys



Jan. 7, 1964 E. J. KoHLMr-:YER ETAL 3,116,997

PROCESS FOR MAKING ALUMINUM-'SILICON ALLOYS Filed Dec. 11, 1959 2 Sheets-Sheet 1 Tull- Jan. 7, 1964 E. J. KoHLMEYl-:R ETAL 3,116,997

PROCESS FOR MAKING ALUMINUM-SILICON ALLOYS Filed Dec. l1, 1959 2 Sheets-Sheet 2 United States Patent O 3,lli6,997 PRQCEdS FR MAJKHNG ALUMHNUM- SHLJHCN .ltlblf' Ernst liristus Kohlmeyer, Berlin-Grunewald, and Eoliennes Schmitt and Hubert Wittner, both of Rheinfelden,

Germany, to Aluminium-lndustrie- Aktien-Gese isehait, Chippis, Switzerland, a joint-steelt company oi Switzerland llliled Dec. ll, i959, Ser. No. 858,923 Claims priority, application Switzerland Aug. 31, 1959 l Claims. (Cl. l5- lm For many decades aluminum-silicon alloys have been obtained by reduction of oxidic raw-materials of aluminum and silicon by means of carbonaceous reduction means in electrotherimal furnaces. In nature there are inexhaustible raw-material sources which are found at many places. The raw-materials are mostly lraolin, clay, and the like.

The process for making aluminum alloys electrothernsally is presently carried out in furnaces with vertical electrodes, and has been empirically developed. Nevertheless, inV the course of recent years the aluminum-content of the aluminum-silicon alloys obtained has gradually bcen raised. About 30 years ago an aluminum-content of 65% was considered as the upper limit which could be reached. `l'oater on it was raised up to 70%, but it has been considered impossible to obtain an aluminum content higher than 7G-72% (calculated on the sum Al-l-Si). E-orts to ge an alloy with a higher content of aluminum yhave always resulted in a product which was composed of aluminum carbide and aluminum-silicon alloy.

@ur present invention relates to a process of making substantially carbon-tree aluminum-silicon alloys with more than 72% aluminum by electrothermal reduction ol raw-mater ls containing alumina and silica.

We have found the production of carbon-free or carbide-irse, aluminum-silicon alloys by direct reduction ot the raw 4materials in an electric furnace represents chiefly a thermal problem which cannot be solved in the electrothermal furnaces generally used at present. As the temperature in the electric furnaces such as they are presently constructed is high and fluctuating, there are in these furnaces temperature-conditions wiich cannot be kept under control. At uncontrolled high temperatures carbides are formed by reaction of carbon or carbon-containing gases with alumina and silica or with aluminum and silicon as soon as one tries to produce aluminumsilicon alloys `with a higher content of aluminum than 65%. l

v-,f'e have found that aluminum-silicon alloys with 72% aluminum and more can be obtained if the following conditions are simultaneously observed:

(l) The reduction of the oxides ot aluminum and of silicon must be carried out Within a narrow temperature zone in which the speed of the reactions which lead to the formation of the aluminum-silicon alloy is great, in which already-formed carbides of aluminum and of silicon react with alumina and silica to form metal, and in iv .ch the volatilization losses of the aluminum and of the silicon are still small. 'lhis zone of temperatures extends from about 2653* C. up to about 2280'o C. When observing an upper temperature limit ol about 2260" C. the vapori- Zation losses are still small; at over about 2260* C. the volatilization of the aluminum and `of the silicon rises rapidly.

(2) rhe raw-material mixture must lbe heated as quickly as possible to the temperature zone which is necessary for the 4metal liberation in order to pass so quickly lbeyond the temperature Zone of 1690-2006" C. (which is favorable fort e formation of earbides) that virtually no formation of carbides tal-ies place.

(3) rEhe formed aluminum-silicon alloy must be reioved as soon as possible after its formation continu- 3,ll,97 Patented Jan. 7, 19h24 ously from the hot reduction-room and cooled down to a temperature below said W C. ligure at which the carburization by reaction with carbon or carbonaceous gases under formation of carbides can no longer take place. Care must therefore be taken that the aluminumsilicon alloy liovvs as quickly as possible out of the reduction-room.

The simultaneous fulfillment of the three above mentioned conditions can Ibe attained by the process according to our invention.

According to the process of our invention the raw-material mixture is charged in such a manner that overheating is prevented in the reduction-room; in other Words, the manner in which the raw-material mixture is added is such that the temperature in the reduction-room does not rise to a level at which the Volatilization losses of aluminum and of silicon are too great, that is to say to a temperature over 226W C. Preferably the upper limit of the temperature is kept under this temperature of 2290" C. The purpose can be attained by keeping the temperature of the raw-material mixture when charged well enough under the upper admissible limit in the reductionroom. The raw-material mixture may be, for instance, at room-temperature; though, depending on the size and the construction of the furnace, it may be advantageous to charge the raw-material in a pre-heated state. vFurthermore, it is necessary to charge the raw-material mixture substantially continuously, ie., either continuously or at short time intervals.

Moreover care is taken, according to our invention, that the formed molten aluminum-silicon alloy leaves the hot zone of the reduction-room continuously without trickling or flowing through a less hot layer of raw-material mixture in which aluminum-carbide could be formed. vWith the known electrothermal processes for making aluminum-silicon alloys the formed alloy flows through a still unreacted layer of charge, in which layer the danger 4of carburization, that is to say oi `carbide formation, exists.

According to our invention the reduction Zone is separated from the lower aluminum-silicon alloy collecting room by a room in which the aluminum-silicon alloy becomes cooled as quickly as possible to a temperature (for instance between its melting point and 1600 C.) at which a reaction between the aluminum-silicon alloy and carbon with formation of carbide can no longer take place. The carbon could be supplied either by the wall of the collecting-room or by particles of the raw-material mixture which become dragged along out of the reductionroom by the aluminum-silicon alloy flowing down or which simply fall out of the reduction-room into the collecting-room. Consequently there must be a separating room between the reduction-room and the collectingroom. Furthermore, when carrying out the process according to our invention heat is desirably supplied to the reduction-room by electric resistance heating. Most advantageously the reduction-room is surrounded by a conducting mass, for instance, of nely divided carbon, through which an electric current is conducted.

Also heating-rods or other heating conductors may be used. Care should be taken that the heat be supplied laterally to the reduction-room, but one may also additionally dispose heating-rods through the raw-material mixture itself in the reduction-room. Also inductive heating may be utilized. There are many possibilities for heating the raw-material mixture to the reaction temperature Without using electric-arc electrodes. lt is essential that there be a sufcient heat supply to keep the most favorable temperature zone.

The speed of charge of the raW-imaterial mix-ture must be so controlled that the heat supplied to the furnace is consumed by the quick heating and the melting of the 3 rawdnaterial mixture and by as well as by compensating reaction-products carried ofi. En order to maintain the favorable temperature range we have found that it is advantageous to charge the raw-material mixture in such a manner that the reduction-room be always kept full.

The physical conditions of the raw-material mixture to be charged has, of course, an influence on the progress of the reaction. Normally with such processes, the rawmaterial mixture is charged in the form of briquettes. When carrying out the process according to our invention, especially uniform progress of the reaction with a maintained heat balance is obtained by charging the rawmaterial as pellets of about 35-20 mm. diameter. To .make the pellets, the raw-material mixture is, for instance, mixed with Ztl-25% of water and poured in a known manner on a rotating disc-plate or treated in a rotating drum. lt may be advantageous to add to lthe raw-material mixture a few percents of sulphur as sulphides, sulphatcs, or other sulphur compounds in order to obtain a quicker melting.

At the suitable temperature of, for instance, about 2160" C., there taires place a quick inciting as well as a quick cduction. When continuously charging `further raw-material mixture directly onto the surface of the mixture which has just been charged `and is continuously `moving down, the new charge acts to some extent as a coolant on the melt, thereby preventing overheating. Since a melting substance cannot be heated over its melting point, a constant temperature in the melting-room (reduction-roem) yitself will always prevail.

rPhe reduction-room may be advantageously disposed vertically above the collecting-receptacle. With such a disposition the aluminum-silicon alloy can reach the collecting-receptacle most quickly--by a free fall, for example. If, for instance, the reduction zone is located in the lower part of `a reduotion-crucible which is disposed at a sulhcient distance above the collecting-room, so that the molten aluminum-silicon alloy can cool down sufficiently during its free fall after leaving the reduction zone, and if, throw-fh this disposition the collected metal be Suthciently separated in space from the perforated bottom of the Crucible, conditions `are attained which allow the carrying out of the process according to our invention.

rl`he purpose of the separating-room under the reduction-room is to remove the molten aluminum-silicon alloy as quickly as possible from 'Contact with the reacting charge. Of course, `for instance, :perforated non-reactive .intermediate bottoms may be disposed in this `separatingroom, which bottoms prevent an excessive reflection of heat from the reduction-room downwards or fully prevent such a rellection. The separating-room may be also partly iilled with a bed of lumps or grains of a material (for instance lumps of corundurn) which has a poor thermal conductivity and does not react with the molten aluminum-silicon alloy. Such lumps or grains may be disposed on perforated intermediate bottoms.

Furthermore the mounting of other types of battles between the reduction-room and the collecting-room may be advantageous.

instead of a vertical arrangement one may choose a disposition at which the aluminum-silicon alloy flows obliquely into the collecting-receptacle. With such a disposition one should suitably take care that the wall along which the aluminum-silicon alloy flows down has such a composition or such a temperature that it cannot react with the aluminum-silicon alloy and form carbide.

in some cases it may be advantageous to dispose cooling elements in the separating-room in order to accelerate the cooling of the amminum-silicon alloy. These cooling elements may, for instance, be rods made from inert material with a iiller of copper, the heat being withdrawn from these rods outside the furnace by means of a cooling liquid.

the reduction of the oxides for the losses through the lt may be "he advantageous to provide the collectinm receptacle with a device allowing either a heating or a cooling, so tl .t the tcm erature which is necessary `for ing t c heat-bounce can oe controlled.

When `carrying out the process according to our invenone may, of course, use as `a collecting-receptacle a channel or a pan from which the aluminum-silicon alloy flows continuously into .er receptacle or eyen into a casting device, for instance into a pig-casting machine.

As the raw rnate-rial mixture orust be heated in as short a time es pos Ule to the reduction temperature range must pass as quickly as possible through the ten` )cinture zone of i606 to ZOG' C. (which is favorable for the formation of carhides) it is adv' able at the beginning of the operation to heat the red on-rooin to a temperature of for instan e 2G50" to 2200" C. before charging the raw-material n Gf course, the process according to our invention can also be applied when one `wishes to malte an aluminum-silicon alloy which contains less than 72% aluminum, for instance down to 65% alu tum or even 60% aluminum. Even though aluminum-silicon alloys u i contain more than 60% or even more than 65% aluminum in the usual electric turna es can be obtained by known processes, the process according to our invention lis superior to the known processes Afor mal-:ing such aluminum-silicon alloys, as the reduction can be much better controlled so that the process is also superior with respect to the expenditure of energy.

rEhe aluminum-silicon alloys obtained accor 'ng to the process of our invention may have a rather high content of iron (for instance as in the `ferro-Silico-aluminum), of titanium, and of other elements.

ln the drawings:

HG. l is a vertical sectional view orf one form of appt.- ratus embodying the invention;

FlG. 2 is a horizontal sectional view thereof;

PEG. 3 is a vertical sectional view of a modification; and

FG. 4 is a horizontal sectional view thereof.

In FlGS. 1 and 2 there is illustrated an embodiment of the invention in the form of a small experimental furnace of a capacity of about S0 liilowatts. The iron furnace-shell Il is provided with a lining 2 of tire clay and filled with finely divided carbon 3. Two parallel walls 4 built up with magnesite bricks limit the inner room at both sides of the current-supply nipples 5 made of graphite and of the water-cooled steel electrodes 6. A cylindrical Crucible '7 made of graphite is disposed in the furnace at such a height that its lower part which serves as the reduction room is at the same height as the current-supply nipples 5. The Crucible '7 is formed with a perforated bottom as shown, and is surrounded by a cylinder il made from electrode-carbon, which cylinder Lserves to transfer the heat uniformly to the reduction room. Under the reduction Crucible 7 there is disposed an intermediate vessel 5B which is also made of graphite and has a perforated bottom. Under the intermediate vessel there is a collecting receptacle llt) made of claygraphite. The lower part of the receptacle t@ projects downwards out of the furnace and is surrounded by a mantle lll of fire clay powder which protects it against a too great cooling. On the reduction Crucible 7 there is laid a covering plate l2 made of electrodecarbon with a central hole for the charging.

As can be seen the reacting charge in the Crucible '7, which is maintained by resistance heating at a temperature of for instance 2050 to 2200 C., is separated in space from the aluminum-silicon alloy which collects in the collecting vessel liti. The aluminum-silicon alloy formed in the Crucible flows out of the reaction chamber through the vessel 9 into the collecting receptacle lil (a part of which is disposed outside the furnace) before it can carburize. The temperature of the collecting receptacle is only about 900 C., that is about l150 C. to

t3 1300 C. below the temperature of the reduction Zone, so that the aluminum-silicon alloy does not take up carbon in the collecting receptace.

FTGS. 3 and 4- show another example of a small experimental furnace embodying the invention. The arrangement is the same as in the experimental furnace shown in FiGS. 1 and 2, except that the two walls from magnesite bricks are not used; but current connections are provided for the additional heating by means of the graphite heating-rods 13.

ln furnaces such as illustrated we succeeded in obtaining among others an aluminum-silicon alloy with 76.0% aluminum and 23.1% of silicon; the remainder being composed of the usual impurities (iron, titanium, and so on) which occur in electrothermally produced aluminumsilicon alloys. This aluminum-silicon alloy was obtained from a mixture of the following raw-materials:

27.4% raw kaolin 23.8% burnt kaolin 23.6% alumina 2.1% silica (quartz) 23.1% charcoal The mixture was pelletized to grains of 15 to 20 mm. cross-section, which sustained without damage a fall from a height of 1.5 to 2 meters.

Before introducing7 the raw-material mixture, the reduction crucible 7 was heated to a temperature of 2100o C. to 2200 C. and kent at this temperature during the continuous charging of raw-material mixture.

Since certain changes in carrying out the above process and in the constructions set forth, which embody the invention, may be made without departing from its scope, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A process of making aluminum-silicon alloys by electrothermal reduction of oxidic raw materials of aluminum and silicon, which comprises heating the walls of a graphite-walled reduction chamber to an elevated temperature of not less than 2050D C. and not more than 2200 C. through the action of a current passing through a conducting body of finely divided electrically resistant carbon surrounding the reduction chamber, charging the reduction chamber whose walls have been so heated with a mixture comprising oxidic raw materials of aluminum and silicon and carbonaceous reducing agents, maintaining the charge in said chamber at a temperature of about 2050o C. to about 2200o C. to reduce the oxidic raw materiais to molten aluminum-silicon alloy substantially free from carbides and without substantial volatilization, causing the molten alloy substantially as soon as it is formed to escape by free fall from said reduction chamber directly into a zone substantially cooler than said reduction chamber and to leave the hot zone of the reduction chamber continuously without flowing through a less hot layer of raw material mixture in which aluminum carbide could be formed, while retaining the solid part of the charge in said reduction chamber, and collecting the molten alloy escaping into said cooler zone almost immediately therafter in a collecting chamber maintained at a temperature sufficient to maintain the temperature of the alloy while in said collecting chamber below 1600 C.

2. A process as set forth in claim 1, wherein the molten alloy in said collecting chamber is continuously Withdrawn therefrom.

3. The process as set forth in claim 1, wherein the temperature in the reduction chamber is maintained at not over about 2200 C. by substantially continuous charging of the reduction chamber.

4. The process as set forth in claim 1, wherein the heat is supplied to said reduction chamber mainly through the current conducted through said body of carbon.

References Cited in the tile of this patent UNITED STATES PATENTS 1,512,462 Haglund Oct. 21, 1924 2,636,864 Wroughton et al Aug. 17, 1954 2,697,127 Poland Dec. 14, 1954 2,755,178 Rasmussen July 17, 1956 2,825,641 Beall et al. Mar. 4, 1958 OTHER REFERENCES Kroll et al.: Trans. of the Electrochem Soc., September 1949, vol. 96, No. 3, pages 158169. 

1. A PROCESS OF MAKING ALLUMINUM-SILICON ALLOYS BY ELECTROTHERMAL REDUCTION OF OXIDIC RAW MATERIALS OF ALUMINUM AND SILICON, WHICH COMPRISES HEATING THE WALLS OF A GRAPHITE-WALLED REDUCTION CHAMBER TO AN ELEVATED TEMPERATURE OF NOT LESS THAN 2050*C. AND NOT MORE THAN 2200*C. THROUGH THE ACTION OF A CURRENT PASSING THROUGH A CONDUCTING BODY OF FINELY DIVIDED ELECTRICALLY RESISTANT CARBON SURROUNDING THE REDUCTION CHAMBER, CHARGING THE REDUCTION CHAMBER WHOSE WALLS HAVE BEEN SO HEATED WITH A MIXTURE COMPRISING OXIDIC RAW MATERIALS OF ALUMINUM AND SILICON AND CARBONACEOUS REDUCING AGENTS, MAINTAINING THE CHARGE IN SAID CHAMBER AT A TEMPERATURE OF ABOUT 2050*C. TO ABOUT 2200*C. TO REDUCE THE OXIDIC RAW MATERIALS TO MOLTEN ALUMINUM-SILICON ALLOY SUBSTANTIALLY FREE FROM CARBIDES AND WITHOUT SUBSTANTIAL VOLATILIZATION, CAUSING THE MOLTEN ALLOY SUBSTANTIALLY AS SOON AS IT IS FORMED TO ESCAPE BY FREE FALL FROM SAID REDUCTION CHAMBER DIRECTLY INTO A ZONE SUBSTANTIALLY COOLER THAN SAID REDUCTION CHAMBER AND TO LEAVE THE HOT ZONE OF THE REDUCTION CHAMBER CONTINUOUSLY WITHOUT FLOWING THROUGH A LESS HOT LAYER OF RAW MATERIAL MIXTURE IN WHICH ALUMINUM CARBIDE COULD BE FORMED, WHILE RETAINING THE SOLID PART OF THE CHARGE IN SAID REDUCTION CHAMBER, AND COLLECTING THE MOLTEN ALLOY ESCAPING INTO SAID COOLER ZONE ALMOST IMMEDIATELY THEREAFTER IN A COLLECTING CHAMBER MAINTAINED AT A TEMPERAATURE SUFFICIENT TO MAINTAIN THE TEMPERATURE OF THE ALLOY WHILE IN SAID COLLECTING CHAMBER BELOW 1600*C. 